1. Field of Industrial Utility
The present invention concerns a process for removal of acid pollution such as SO.sub.2 and NO.sub.x from flue gases by a radiation method, in particular from heat and power generating stations and an apparatus for removal of SO.sub.2 and NO.sub.x from flue gases.
2. Discussion of the Background
Air pollution caused by gaseous products derived from combustion of coal and fossil fuels products in thermal-electric power stations present a problem of global proportions. A typical power station generating 500 MW of power emits about 3 to 5 tons pollution per hour which has a cumulatively detrimental effect on the environment. Several technologies have been developed to provide effective purification of flue gases.
The removal of acid pollution from flue gases by chemical methods is based on the absorption of acidic impurities in alkaline solutions i.e. lime suspensions. Those wet methods lead to deposition of large quantities of by-products, besides they allow elimination of SO.sub.2 only. Substantial amounts of NO.sub.x still remains in the flue gases and in particular NO, together with Freon compounds is considered to be responsible for the ozone hole. As such, it is necessary to build separate plants for NO.sub.x removal from flue gases. Those plants are based on different principles, mainly on catalytic reduction.
Radiation technology uses a stream of accelerated electrons to generate free radicals. This leads to simultaneous SO.sub.2 and NO.sub.x removal from flue gases.
The radiation method enables elimination of 95% of SO.sub.2 and 80% of NO.sub.x in one plant. In the radiation technology it is important to increase efficiency of reactions which depends on the amount, temperature and composition of the gas mixture. A higher efficiency can be achieved by introducing moisture and a quantity of ammonia before conducting irradiation process as described in Polish Patent No. 153259 and Polish Patent Application No. 284996 filed on 27 Apr. 1990. Those methods are based on simultaneous reactions initiated by radiation and result in the formation of solid products. These products are useful as fertilizers.
The flue gas irradiation in the presence of water aerosols leads to the formation of atomic and molecular radicals and free electrons. Radicals OH., O. and H.sub.2 O. are responsible for oxidation of SO.sub.2 and NO.sub.x to SO.sub.3 and NO.sub.2 and further in the presence of water H.sub.2 SO.sub.4 and HNO.sub.3 are formed. Finally these compounds react with ammonia to form solid products NH.sub.4 NO.sub.3 and (NH.sub.4).sub.2 SO.sub.4, which can be used as fertilizers. The temperature of this process is kept in the range of 65.degree. to 100.degree. C.
Optimalization of temperature, degree of watering and ammonia content depending on gas composition and its flow rate slightly change the efficiency of acid pollution removal from flue gas.
Research has also been conducted into improving the efficiency of radiation methods. Such improvements are based on additional use of electrostatic and electromagnetic fields, which could increase the amount of free electrons and free radials and change the chemical reaction process.
The known method described in Patent DD-243-216A, (87-170590) proposes using, (beside a beam of electrons with energy 5-500 KeV), an electrostatic field having an intensity up to 100 V/cm to reduce the consumption of electrical energy in the process. In this process purifying efficiency is increased. The disadvantage of this method is the necessity of using additional grid electrodes located in the reaction vessel. They are located at a distance of 16 cm one from the other to incorporate the electric field into the space where the reaction proceeds. The solid reaction products and fly ash formed during and after irradiation tend to be deposited on electrodes and block the reaction vessel.
The disadvantage of the method described above can be overcome by a method disclosed in Patent JO-1099-633-A, (89-156548/21) where an irradiation vessel laser beam is utilized (ArF laser with wavelength 193 nm) and CH.sub.3 OH added. CH.sub.3 OH is excited by light to generate OH. radicals which bond NO and SO.sub.2 to solid products and enables their removal. The use of a laser beam has a beneficial effect but industrial application is complicated and expensive. Efficiency is rather low because of limited penetration of UV light in a reaction vessel caused by the presence of water. It is also difficult to obtain good homogeneity of spatial distribution of the light beam, and it is necessary to the use CH.sub.3 OH compound.
The problem of reduction of electrical energy consumption is especially important in industrial scale installations because 2 to 4 per cent of total electrical energy produced in the power station is consumed for purifying flue gases from acid pollution.